1. Field of the Invention
The present invention relates to VHF tuners, and more particularly to a VHF tuner to change feedback capacitance of a local oscillator at low band receiving state and high band receiving state, in which a switching diode in series connection to a capacitor for feedback capacitance is connected to one terminal of a mixing FET in a mixer so that working current of the mixing FET is changed corresponding to changing of the feedback capacitance.
2. Description of the Prior Art
In VHF tuners in general, feedback capacitance between emitter of a local oscillating transistor and the ground (hereinafter referred to as "feedback capacitance") in a local oscillator is changed corresponding to low band receiving state and high band receiving state in VHF region. Referring to FIG. 1, changing of the feedback capacitance in a conventional VHF tuner is performed in that a local oscillating transistor 1 has parallel connection of capacitors 2 and 3 between emitter thereof and the ground, and a switching diode 4 is connected in series to the capacitor 3 and turned on or off by control voltage applied to a switching control terminal 5. That is, the switching diode 4 is turned on at low band receiving state; it is turned off at high band receiving state. As a result, the feedback capacitance at low band receiving state becomes composite capacitance of the capacitors 2 and 3 since the capacitor 3 is grounded through the switching diode 4. On the contrary, the feedback capacitance at high band receiving state becomes capacitance value of only the capacitor 2 since the switching diode 4 is turned off. Output of the local oscillating transistor 1 is generated on changing the feedback capacitance in above-mentioned manner and then transmitted through a capacitor 6 to a mixing FET 7. The mixing FET 7 performs mixing process with high frequency signal entered from a high frequency input terminal 8, and intermediate frequency signal is outputted through an intermediate frequency output terminal 9. Furthermore, reference numeral 10 in FIG. 1 designates a resonance circuit connecting terminal, numeral 11 a local oscillation voltage source terminal, numeral 12 a mixer voltage source terminal, numeral 13 a choking coil, numerals 14-23 resistors, and numerals 24-27 capacitors.
In the example of prior art described referring to FIG. 1, the feedback capacitance is changed at low band receiving state and high band receiving state, thereby oscillation frequency of the local oscillator can be changed corresponding to both bands. Since the switching diode 4 to change the feedback capacitance is grounded directly, in order to turn the switching diode 4 off, negative potential must be applied to the switching control terminal 5 or otherwise a prescribed reverse bias circuit must be installed without grounding the diode 4 directly. Although it is preferable that bias of the mixer be changed corresponding to low band receiving state and high band receiving state so as to prevent distorsion from occurring in the mixer during mixing process, bias of the mixer is fixed in the example of prior art shown in FIG. 1.